1. Field of the Invention
The present invention relates to an image data encoding/decoding apparatus for use in an image data communication apparatus such as a facsimile apparatus.
2. Description of the Related Art
In an image data communications apparatus such as a facsimile apparatus, image data are derived by successively scanning a document in successive scan lines, with the respectively corresponding sets of data obtained for the scan lines thereby being referred to in the following as image data line portions. Normally, each bit of the data represents a black or white picture element state. Before being transmitted to a distant facsimile apparatus, the amount of data that must be transmitted is reduced by encoding the image data, using a standard encoding format such as the MH, MR, or MR.sup.2 format. The encoding may be one-dimensional (i.e. based on black/white transitions within each line) or two-dimensional. Two-dimensional coding includes a plurality of encoding modes, and is based on relationships between positions of transitions within a line and positions of transitions in the preceding (already encoded) line, which functions as a reference line. At the receiving facsimile apparatus, the encoded data are decoded to recover the original image data.
There is at present a requirement for types of facsimile apparatus which provide a high speed of processing and are capable of concurrent processing of respectively different streams of image data that are to be encoded, or streams of encoded data that are to be decoded to recover image data. With such a facsimile apparatus, image data and encoded data can be temporarily stored in memory. This enables a user, for example, to rapidly generate image data by scanning a document, without the need to wait for the system to execute transmission of the resultant image data during the scanning operation. In addition if concurrent processing is possible, then for example image data for respectively different documents can be read out from memory and encoded at the same time, to be stored in memory and subsequently transmitted, or encoded data for different documents can be decoded at the same time, and the results stored in memory to be subsequently sent to a printer.
FIGS. 1 and 2 show respective prior art examples of arrangements for concurrent encoding of two different streams of image data, and for decoding two different streams of encoded data. In FIG. 1, an encoder A receives an input data stream designated as image data #1, as a serial bit stream, while an encoder B similarly receives an input data stream designated as image data #2. As shown, the encoders A and B are formed of transition point detection circuits 1a, 2b respectively, which receive as inputs the image data #1 and image data #2 respectively, mode judgement circuits 2a, 2b which receive outputs from the transition point detection circuits 1a, 1b respectively, table look-up circuits 3a, 3b which receive outputs from the mode judgement circuits 2a, 2b, and encoded data output registers 4a, 4b which receive outputs from the table look-up circuits 3a, 3b. The respective outputs from the encoded data output registers 4a, 4b are supplied to an encoded data selector 5, which is controlled by an encoded data selection signal to periodically select one of these outputs to be transferred, e.g. to be stored in memory. In the encoder A, the transition point detection circuit 1a detects transitions between the black/white states in the image data #1, the mode judgement circuit a judges the encoding modes which are applicable, and based on the modes and the detected positions of transitions , corresponding code values are obtained from the table look-up circuit 3a and temporarily held in the encoded data output register 4a. The image data #2 is similarly encoded by the encoder B.
In FIG. 2, the decoders A and B are respectively formed of encoded data input registers 6a, 6b which receive the streams of encoded data #1 and encoded data #2, while outputs from the encoded data input registers 6a, 6b are supplied to table look-up circuits 7a, 7b respectively, outputs from the table look-up circuits 7a, 7b are supplied to respective mode judgement circuits 8a, 8b, and outputs from the mode judgement circuits 8a, 8b are supplied to respective image data restoration circuits 9a, 9b. In the decoder A, the encoded data are temporarily held in the encoded data input register 6a, and the original image data are recovered by the image data restoration circuit based on results obtained from the table look-up circuit 7a and mode judgement circuit 8a. The encoded data #2 are similarly processed by the decoder B. Output image data from the decoder A or decoder B are selected by the image data output selector 10, e.g. to be transferred to a memory.
However such a prior art type of image data encoding/decoding apparatus has the basic disadvantage that it is necessary to provide two complete encoder circuits and two complete decoder circuits in order to achieve concurrent encoding or concurrent decoding of separate streams of image data or encoded data. Thus, the overall system scale of a facsimile apparatus using such an image data encoding/decoding apparatus will be increased by comparison with a facsimile apparatus which does not have such a concurrent encoding/decoding capability.